I tried to log into iCloud.com yesterday, and I was greeted with a little security box asking me to verify my email address. When I entered my Apple ID email address, I got a peculiar and irritating error: "This email address is already being used as an alternate email address. Remove this email address from your alternate addresses and try again."

Here's the steps I used to fix the problem. Note that I specify in some spots to press "Done" — this is important, because if you try to edit too many things at once without pressing done, changes don't apply correctly and you'll get logged out and have to log back in.

Enter a different email address, and then enter the verification code (from your email inbox) to proceed. At that point, this other email address will become your Apple ID, which like me you probably don't want. The rest of these steps are to change back to the Apple ID email address you were using in the first place.

Click "Edit" at the right of the "Account" section. Remove your original Apple ID email address as an alternate "reachable at" address. Click done.

Click "Edit" at the right of the "Account" section again. Change your Apple ID back to your original email address you want to be using. Enter the verification code from the email you get sent. Press done.

Take a look to see if the "different" email address you were using is showing up as a "Reachable At" email address. Depending on whether you want that email associated with iMessage, keep or remove it from that list.

I got kicked out of the website a couple of times while doing this, all from trying to do too many (ahem, more than one) things at once. If you get kicked out, just log back in and carry on where you left off.

Logically, it seems that by switching back to your original-icloud-email, you would recreate the problem that you were solving. Because that original-icloud-email is still an alternate email address for another iCloud account.

In my situation, the primary email address was somehow also an alternate for the same account itself, which was causing the problem. So temporarily switching to a different email, removing the original address from the "reachable at" list, and re-setting/re-adding the original as the primary worked for me.

If you have the address set as a "reachable at" on a different account, you may need to log in to that different account and remove it. But in this post, the bug was that the primary email address was also a an alternate for the same account.

Ah that is unfortunate! Hmm... if you think any iOS device may be signed on to the account, you could try sending an iMessage to that email address, and see if you hear a beep or get a reply? Or call via FaceTime.

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